Investigations into the phytoplankton of the Bay of Biscay in July 2006 revealed an extensive bloom of an unidentified haptophyte species. High-resolution sampling techniques identified the region in the vicinity of the mouth of the Loire estuary as an area of high (106–107 cells l−1) cell densities of the haptophyte in a thin sub-surface layer. This organism dominated the phytoplankton population regionally. Measurements of the vertical distribution of horizontal shear showed that the population, located at ca. 20 m depth, was confined to a layer lying between two density discontinuities with high shear. Physical measurements indicated that the population was being advected southeastwards, along the coast. Seaward spreading of the population was also noted during the survey. Horizontal diffusion could be derived from the changes in the offshore gradient in the population's distribution with time, with estimates of Kx, the horizontal diffusion coefficient, of the order of 100 m2 s−1. The magnitude of horizontal dispersion is compared with that of horizontal advective flux, vertical dispersion and growth.